The increase in temperature in the chromosphere, transition region and corona must be explained by some non-radiative form of energy. If radiation was the only energy being transported through the atmosphere, the temperature should decrease further out from the core, as more energy is escaping. An extra heating mechanism must exist, which will provide energy for atoms to become excited and decay. This process of absorption and decay acts as a thermostat for the lower chromosphere, and is mainly provided by hydrogen atoms (providing the red chromosphere during eclipse totality in Hα). When the supply of hydrogen atoms runs out the temperature increases sharply, marking the transition zone. A further thermostat is provided in the corona by the large number of highly ionized atoms. This provides a second temperature plateau.
One of the most controversial areas in solar physics at the moment is the necessity of the existence of a temperature minimum at all (Figure 1.10). The rise in temperature around 500 km is a direct result of the VAL model (Vernazza et al. 1981) and later in the FAL model (Fontenla, Avrett, & Loeser 1993) to explain the chromospheric emission in EUV lines and was generally accepted for many years. Ulmschneider (1970) suggested short period (~ 30 s) waves shocking in the chromosphere to explain this, but no observation evidence could be found. In contrast, previous stellar-like classical radiative equilibrium (RE) models (e.g., Gustafsson & Jorgensen 1994), and the HOLMUL model (Holweger & Müller 1974) showed no need for a temperature rise.
Modelling of internetwork grains (Carlsson & Stein 1992; 1995; 1997) suggested the temperature rise was purely a inversion technique artifact. They suggested that acoustic shocks in the internetwork may intermittently heat the chromosphere, without significantly increasing the average temperature. However, at UV wavelengths, they may be sufficiently bright to as to require a temperature rise when inverted as in the VAL and FAL models. The idea of a cool atmosphere with intermittent shocks also agrees with models derived from the CO line formation (e.g., Ayres 1981). Other authors (e.g., Kalkofen, Ulmschneider, & Avrett 1999) question the validity of this model.
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